Pharmaceuticals for neuropathic pain

ABSTRACT

This invention relates to pharmaceuticals for neuropathic pains comprising an mGluR1 receptor antagonist for systemic administration. Drugs efficacious in treating various neuropathic pain can be provided by the invention.

TECHNICAL FIELD

[0001] This invention relates to novel medical uses as pharmaceuticalsfor neuropathic pain by systemically administrating a compound having anmGluR1 receptor antagonistic activity.

BACKGROUND OF THE INVENTION

[0002] Neuropathic pain is intractable pain caused as a result of afunctional abnormality of the peripheral or central nervous system.Neuropathic pain manifests itself due to neurological disordersaccompanying various causes such as wound, infection, cancer, ischemiaand metabolic disorders including diabetes mellitus. Though there aremany unclear points on the mechanism of neuropathic pain, it isconsidered that abnormal continuous firing of sensory nerve and the likeare the cause. Typical symptoms of neuropathic pain include allodinia,hyperalgesia, hyperesthesia and the like. Their symptoms includecharacteristic pains expressed as “like burning”, “like stinging”, “likeelectrical shock” and the like. It is known that analgesics,particularly narcotic analgesics and the like, which are effective forgeneral nociceptive pains are hardly effective for neuropathic pain (TheLancet, 353, 1959-1966, 1999). For example, it is known that morphinehas a strong analgesic effect on nociceptive pains but does not show asufficient effect on neuropathic pain. Consequently, this resistance tomorphine is used in diagnoses as a main characteristic of neuropathicpain (Advances in Medical Science, 189(10), 751-755, 1999). It isconsidered that the ineffectiveness of morphine for neuropathic pain isdue to degeneration of inhibitory neurons and reduction of opiatereceptors caused by neurological changes of nerves (Recent Brain andNeuroscience Series vol. 6, Neuroscience of Pains, published by MedicalView, p. 97, 1997).

[0003] Accordingly, it is considered that various factors arecomplicatedly involved in the generation and maintenance of neuropathicpain. As the therapeutic methods, neurosurgery treatments such as nerveblock and epidural spinal cord electric stimulus (Advances in MedicalScience, 189 (10), 757-762, 1999), tricyclic antidepressants (Clinicaland Drug therapies, 18 (7), 643-646, 1999), intrathecal administrationof baclofen (Functional Cerebral Nerve Surgery, 33, 45-49, 1994) and thelike have so far been used. However, since a safe and effectivetherapeutic method has not been established, concern has been directedtoward the development of a therapy effective for neuropathic pain.

[0004] Glutamate is a main excitatory transmitter that is present insensory nerve fibers and mediates sensory information to the spinalcord. Peripherally evoked nociceptive signals are sent to supraspinalregions through non-NMDA, NMDA and mGluR5 in the spinal cord. Also, ithas been reported that NMDA receptor antagonists and AMPA receptorantagonists improve reduced pain thresholds in a neuropathic pain model(Br. J. Pharmacol., 122, 1478-1482, 1997). Based on the above, it isconsidered that in the spinal cord glutamate may be released excessivelyin the state of neuropathic pain.

[0005] On the other hand, the following reports have been published onthe involvement of mGluRs in neuropathic pain.

[0006] Reference 1 (Neuroreport, 9, 731-735, 1998) reported thatintrathecally administered antibodies for mGluR1 and mGluR5 before and24 hours after surgical operation inhibited a development of coldhyperalgesia but did not inhibit a development of mechanical allodyniain rats.

[0007] Reference 2 (Pain, 77, 59-66, 1998) reported that intrathecaltreatment (twice-daily injections on post-operative days 0-8) of a GroupI antagonist ((S)-4CPG: (S)-4-carboxyphenylglycine) attenuated coldhyperalgesia on the post-operative days 4 and 8. But it did not have aneffect on that on the post-operative days 12 and 16. In addition, italso inhibited a development of mechanical allodynia on thepost-operative days 4 and 8, but it did not have an effect on that onthe post-operative days 12 and 16. On the other hand, twice-dailyintrathecal injections of (S)-4CPG on post-operative days 8-11 did notinhibit development of lowered pain threshold at any day of the test.Therefore, it was discussed that Group I mGluRs are involved in thedevelopment, and not the maintenance, of mechanical allodynia and coldhyperalgesia.

[0008] The authors of the references 1 and 2 have confirmed prophylacticeffects of the intrathecal administration of mGluR1/5 antagonist andantibody for mGluR1 in nerve ligation-induced neuropathic pain animalmodels. However, since the reference 2 has concluded that mGluR1/R5antagonist did not improve the well-established mechanical allodynia orcold hyperalgesia, it has been considered that mGluR1 and/or mGluR5antagonist has an prophylactic effect, which means the prevention ofpain threshold reduction after nerve injury, but does not have atherapeutic effect, which means improving lowered pain thresholds to thenormal level.

[0009] In addition, these references 1 and 2 neither disclose norsuggest the therapeutic effect of systemically administered mGluR1antagonist on neuropathic pain.

[0010] In these references 1 and 2, intrathecal administration is usedfor the evaluation of drugs in neuropathic pain models. Since it isknown that mGluR1 receptor is involved in the nociceptive signaling inthe spinal cord, it is likely that the mGluR1/R5 antagonists andantibody for mGluR1 were intrathecally administered in the references 1and 2. However, these mGluR1/R5 antagonists and antibody for mGluR1 didnot show the therapeutic effect on neuropathic pain even by theirintrathecal administration, which is thought to be the most efficientadministration route to deliver drugs to the action sites.

[0011] On the other hand, it has been reported that the mGluR1 receptoris highly expressed in the thalamus and exists particularly on relayneurons in the thalamus, which transmit noxious signals to the cerebralcortex (Neuron, 9, 259-270, 1992; Neurochem. Int., 24, 451-458, 1994).

[0012] Therefore, the present inventors have considered that it isnecessary to block mGluR1 in the thalamus in addition to mGluR1 in thespinal cord to obtain a sufficient therapeutic effect for neuropathicpain, and have attempted to evaluate the therapeutic effect of mGluR1antagonists in neuropathic pain models by systemic administration.

[0013] The relationship between mGluR1 in thalamus and neuropathic painhas not been known, and there has been no suggestion from which thetherapeutic effect of an mGluR1 antagonist on neuropathic pain by itssystemic administration can be predicted.

DISCLOSURE OF THE INVENTION

[0014] The object of the invention is to provide excellent systemicallyactive pharmaceuticals for treatment of neuropathic pain.

[0015] With the aim of achieving this object, the inventors haveconducted studies based on our own ideas and found that an mGluR1antagonist has therapeutic effects in various neuropathic pain models,thereby accomplishing the invention. STZ (streptozotocin)-induceddiabetic mice are used as a neuropathic pain model in which the pain isinduced by diabetes-related neurological disorders, and spinal L5/L6nerve-ligated rats are used as a neuropathic pain model in which thepain is induced by a compression-induced nerve damage (Pain, 50,355-363, 1992). When the effect of the mGluR1 antagonists on loweredpain thresholds by their systemic administration was examined usingthese models, it was found that the mGluR1 antagonists have an effectwhich cannot be predicted from the technical levels so far known, namelythat the mGluR1 antagonist significantly recovers lowered painthresholds in these models (therapeutic effect).

[0016] The present invention makes it possible to providepharmaceuticals for neuropathic pain, which are easy for patients totake and have an efficient therapeutic effect with less side effects.

[0017] Particularly, the invention relates to the following items.

[0018] A pharmaceutical composition for systemic administration for usein treating a neuropathic pain, which contains a compound having mGluR1antagonistic activity in an amount effective for improving theneuropathic pain and a pharmaceutically acceptable carrier; preferably,the pharmaceutical composition wherein the neuropathic pain is aneuropathic pain induced by diabetes or compression of nerves;

[0019] more preferably, the pharmaceutical composition wherein theneuropathic pain is a neuropathic pain induced by diabetes; and

[0020] more preferably, the pharmaceutical composition wherein thesystemic administration method is oral administration.

[0021] Further preferably, the pharmaceutical composition wherein thecompound having mGluR1 antagonistic activity is a compound havingsufficient mGluR1 antagonistic activity for expressing neuropathic painimproving effect by systemic administration; and

[0022] most preferably, a pharmaceutical for neuropathic pain, whereinthe compound having mGluR1 antagonistic activity is a compound selectedfrom6-amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzoimidazole-2-carboxamidedihydrochloride and(+)-(1R,2S)-6-amino-N-methyl-N-(2-methylcyclohexyl)thiazolo[3,2-a]benzoimidazole-2-carboxamidedihydrochloride.

[0023] It also relates to the pharmaceuticals for neuropathic pain,wherein the compound having mGluR1 antagonistic activity is a compoundhaving an activity of 0.1 μM or less as an IC₅₀ value for the PIformation induced by 100 μM of glutamate.

[0024] The following describes the invention further in detail.

[0025] The neuropathic pain means an abnormal state of pain sensation,in which a reduction of pain threshold and the like are continued, dueto functional abnormalities accompanying damage or degeneration of anerve, plexus or perineural soft tissue, which is caused by wound,compression, infection, cancer, ischemia and the like, or metabolicdisorders such as diabetes mellitus and the like. Illustratively,allodinia (a pain sensation induced by mechanical or thermal stimulusthat does not normally provoke pain), hyperalgesia (an excessiveresponse to a stimulus that is normally painful) and hyperesthesia (anexcessive response to a contact stimulus) are included in neuropathicpain, though not limited thereto.

[0026] Illustrative examples of the neuropathic pain include diabeticpolyneuropathy, entrapment neuropathy, phantom pain, thalamic pain afterstroke, post-herpetic neuralgia, atypical facial neuralgia pain aftertooth extraction and the like, spinal cord injury, trigeminal neuralgiaand cancer pain resistant to narcotic analgesics such as morphine.

[0027] The neuropathic pain includes the pain caused by either centralor peripheral nerve damage. And it includes the pain caused by eithermononeuropathy or polyneuropathy.

[0028] The therapeutic effect means relieving patients from aneuropathic pain by administering a drug after nerve injury, moreillustratively, treating the pain by raising lowered pain thresholds tonormal level.

[0029] The systemic administration means a route of administration bywhich a drug is transferred into systemic circulation so that the drugcan be distributed to the whole body, particularly to the brain, and itsexamples include oral administration, intravenous administration, rectaladministration, intramuscular administration, subcutaneousadministration, sublingual administration and the like.

[0030] The compound having sufficient mGluR1 antagonistic activity fortherapeutic effect on neuropathic pain by systemic administration meansa compound that shows the therapeutic effect on neuropathic pain bysystemically administrating the compound to the patient who diagnosed aspresenting with symptoms of neuropathic pain or neuropathic animals.Even in the case of compounds which have mGluR1 antagonistic activity, acompound which does not have therapeutic effect on neuropathic pain at asubstantially systemic-administrative amount is not included in theinvention. Preferred is a compound having an activity as an IC₅₀ valuefor PI formation induced by 100 μM glutamate, preferably an IC₅₀ valuecalculated by the method described in the following Test Example 1, ofhigh activity than 0.1 μM, more preferred is a compound having theactivity of 60 nM or less.

[0031] The effective amount to treat neuropathic pains by systemicadministration means substantially systemic administrable amount of thecompound having sufficient activity to show treatment effect ofneuropathic pain. The dose of each compound is decided by taking intoconsideration severity of symptoms, pharmacokinetic profile of the drug,administration route, subject to be administered, its age, sex and soon.

[0032] Regarding the compound having mGluR1 antagonistic activity as theactive ingredient of the pharmaceuticals for neuropathic pain of theinvention, its structure is not limited and it may be either a peptidecompound or a non-peptide compound, with the proviso that it is acompound which shows mGluR1 antagonistic activity and has sufficienttherapeutic activity for neuropathic pain by systemic administration.

[0033] The compounds described in the following references or patentscan be cited as examples of such mGluR1 antagonist, of which compoundshaving mGluR1 antagonistic activity and the therapeutic activity forneuropathic pain by systemic administration are included in theinvention.

[0034] Japanese Patent Laid-Open No. JP08-169884, Japanese PatentLaid-Open No. JP11-189596, Japanese Patent Application No.JP2000-102893, WO 96/15100, WO 95/25110, WO 98/06724, WO 99/26927, WO99/44639 and the like.

[0035] Referring to the synthesis methods described in these documentscan produce these compounds.

[0036] The pharmaceutical preparation which contains one or two or moreof the compounds-or salts thereof to be used in the invention as theactive ingredient is prepared using carriers, fillers and otheradditives which are generally used in pharmaceutical preparation.

[0037] The carriers and fillers for the pharmaceutical preparation maybe either solid or liquid, and their examples include lactose, magnesiumstearate, starch, talc, gelatin, agar, pectin, acacia, olive oil, sesameoil, cacao butter, ethylene glycol and the like, as well as othergenerally used materials.

[0038] The administration may be either oral administration by tablets,pills, capsules, granules, powders, solutions and the like, orparenteral administration by injections for use in intravenous,intramuscular or the like injection, suppositories, percutaneouspreparations and the like. Clinical dose is optionally decided by takinginto consideration symptoms and age, sex and the like of each patient tobe treated, but is within the range of generally from 1 to 1,000 mg,preferably from 50 to 200 mg, per day per adult by oral administration,by dividing the daily dose into 1 to several doses per day, or withinthe range of from 1 to 500 mg per day per adult by intravenousadministration, by dividing the daily dose into 1 to several doses perday, or it is continuously administered into a vein within the range offrom 1 hour to 24 hours per day. Since the dose varies under variousconditions as described in the foregoing, a smaller dose than the aboverange may be sufficient enough in some cases.

[0039] As the solid composition for use in the oral administrationaccording to the invention, tablets, powders, granules and the like areused. In such a solid composition, one or more active substances aremixed with at least one inert diluent such as lactose, mannitol,glucose, hydroxypropylcellulose, microcrystalline cellulose, starch,polyvinyl pyrrolidone or magnesium aluminummetasilicate. In the usualway, the composition may contain other additives than the inert diluent,such as a magnesium stearate or the like lubricant, calciumcarboxymethylcellulose or the like disintegrator, lactose or the likestabilizing agent and glutamic acid, aspartic acid or the likesolubilization agent. If necessary, tablets or pills may be coated witha sugar coating or a gastric or enteric coating such as sucrose,gelatin, hydroxypropylcellulose, hydroxypropylmethylcellulose phthalateor the like.

[0040] The liquid composition for oral administration use includespharmaceutically acceptable emulsions, solutions, suspensions, syrups,elixirs and the like and contains a generally used inert diluent such aspurified water or ethyl alcohol. In addition to the inert diluent, thiscomposition may also contain a moistening agent, a suspendion and thelike adjuvants, as well as sweeteners, flavors, aromatics andantiseptics.

[0041] The injections for parenteral administration include asepticaqueous or non-aqueous liquid, suspensions and emulsions. Examples ofthe solutions and the suspensions for use in the aqueous liquid includedistilled water for injection and physiological saline. Examples of thesolutions and the suspensions for use in the non-aqueous liquid includepropylene glycol, polyethylene glycol, olive oil or the like plant oil,ethanol or the like alcohol, Polysorbate 80 and the like. Such acomposition may further contain adjuvant such as an antiseptic, amoistening agent, an emulsifying agent, a dispersing agent, astabilizing agent (e.g., lactose) and a solubilization assisting agent(e.g., glutamic acid or aspartic acid). These compositions aresterilized, e.g., by filtration through a bacteria retaining filter,blending of a germicide or irradiation. Alternatively, they may be usedby firstly making into sterile solid compositions and dissolving them insterile water or a sterile solvent for injection use prior to their use.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a graph showing a reaction latency prolonging effectwhen a compound A (FIG. A) or a compound B (FIG. B) was administered toSTZ-induced diabetes mice.

[0043]FIG. 2 is a graph showing a result of the measurement of painthreshold when the compound A was administered to spinal L5/L6nerve-ligated rats.

BEST MOD FOR CARRYING OUT THE INVENTION

[0044] Next, the invention is described further in detail with referenceto examples, but the invention is not limited to these examples.

EXAMPLE (Production of Tablets)

[0045] Tablets were produced using the following components. ComponentsAmount (mg/tablet) Compound A 200 Cellulose (microcrystalline) 400Silicon dioxide (fume) 10 Stearic acid 5 Total 615 mg

[0046] By mixing and compressing these components, tablets each having615 mg in weight were formed.

[0047] The action of the invention to improve lowered pain thresholds inneuropathic animals was evaluated and confirmed in the following manner.

TEST EXAMPLE 1

[0048] The mGluR1 receptor antagonistic activity of the compounds usedin the invention was confirmed by two test methods.

[0049] (Test Compounds)

[0050] The following tests were carried out using compound A(6-amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzoimidazole-2-carboxamidedihydrochloride) and compound B((+)-(1R,2S)-6-amino-N-methyl-N-(2-methylcyclohexyl)thiazolo[3,2-a]benzoimidazole-2-carboxamidedihydrochloride).

[0051] 1. Measurement of mGluR1 antagonistic activity using cerebellargranule cells

[0052] (Cell Culture)

[0053] The whole brain was removed from each of seven Wistar rats of 7to 8 days of age after birth (Japan S L C), and then the cerebellum wasisolated in an L-15 medium (Gibco) under a stereoscopic microscope. Theisolated cerebellum was cut into pieces using a surgical knife andincubated at 37° C. for 15 minutes in 0.25% trypsin solution (Gibco)containing 750 U/ml of DNase I (Sigma) After termination of the enzymereaction by adding fetal calf serum (JRH Biosciences), the tissue pieceswere centrifuged and the supernatant was removed by sucking. Afteradding 10 ml of a high potassium medium, cells were dissociated by 5 to6 times passage through a plastic pipette. After filtering the celldispersion through a nylon mesh (40 μm pore), viable cells were counted.The cells were diluted in the high potassium medium and then seeded at adensity of 8×10⁵ cells/well onto 24-well culture plates (SumitomoBakelite) precoated with poly-L-lysine. The cells were cultured at 37°C. under a condition of 5% CO₂-95% air.

[0054] High potassium medium: BME (Gibco)+10% immobilized fetal calfserum+25 mM KCl+1% penicillin/streptomycin (Gibco)

[0055] (Phosphatidylinositol (PI) Hydrolysis Assay)

[0056] The PI hydrolysis was measured as described by Aramori et al(Neuron, 8, 757-765, 1992). Cerebellar granule cells were cultured for 2days and then the cells were labeled with myo-[³H]inositol (final 3μCi/ml) at 37° C. overnight. The cells were incubated for 20 minutes inPBS-LiCl solution and then incubated for 20 minutes in PBS-LiCl solutioncontaining each compound. The reaction was terminated by 0.2 M PCA andthe solution was stood at 4° C. for 1 to 2 hours. After adding 2 N KOHand 100 mM EDTA-2Na solution, the plate was centrifuged (2,000 rpm, 5minutes). The supernatant (1 ml) was applied to Bio-Rad AG1-X8 column,washed with GPI solution (5 mM disodium tetraborate, 60 mM sodiumformate) and then eluted with 4 ml of IP3 solution (0.1 M formate, 1 Mammonium formate). The eluate was mixed with a liquid scintillator(Aquasol-2) and the radioactivity in the eluate was determined by aliquid scintillation spectrometer.

[0057] (Results)

[0058] The IC₅₀ value of compound A was 22 nM. Also, the IC₅₀ value ofcompound B was 2.0 nM.

[0059] Thus, it was confirmed that both compounds to be used in theinvention are potent mGluR1 antagonists.

[0060] The following experiments were carried out to confirm these dataobtained from above experiment. 2. Measurement of inhibition activity bymGluR1 α expression cell

[0061] (Cell Culture)

[0062] The NIH3T3 cell lines expressing individually mGluR1 α andmGluR5a were cultured using DMEM containing 10% dialyzed fetal calfserum and 100 units/ml, 0.1 mg/ml of streptomycin sulfate. The CHO celllines expressing individually mGluR2, R4, R6 and R7 were cultured usingDMEM containing 10% dialyzed fetal calf serum, 100 units/ml, 0.1 mg/mlof streptomycin sulfate and 2 mM glutamine.

[0063] (Measurement of Intracellular Calcium Concentration)

[0064] Intracellular calcium concentration of the mGluR5a-expressedcells was measured using a spectrofluorometer as described previously(Nature, 383, 89-92, 1996).

[0065] (PI Hydrolysis Assay)

[0066] Using the mGluR1 α-expressed cells labeled with ³H-inositol,hydrolysis of phosphatidylinositol was measured as described previously(Nature, 383, 89-92, 1996).

[0067] (Measurement of Intracellular cAMP)

[0068] Using cell lines expressing individually mGluR2, R6 and R7, cAMPformation after forskolin stimulation in the presence of IBMX wasmeasured using a cAMP assay kit as described previously (Neuron, 8,169-179, 1992).

[0069] (Results)

[0070] Compound A did not show any agonist or antagonist activity formGluR2, R6 and R7 up to 100 μM. And also, Compound A did not show anyagonist or antagonist activity for mGluR5 up to 10 μM.

[0071] Taken together, it was proved that the compound A does not havethe action upon other groups (Group II and Group III) of metabotropicglutamate.

[0072] Regarding mGluR1 α, the compound A dose-dependently inhibited thePI formation induced by 100 μM glutamate, and its IC₅₀ value was 24 nM.Also, the IC₅₀ value of compound B was 1.7 nM.

[0073] Thus, it was confirmed that the method 1 used cerebellar granulecells could be used for the assay of mGluR1 antagonistic activity as analternative method.

TEST EXAMPLE 2 (STZ-Induced Diabetes Mice Model)

[0074] The test was carried out by modifying a previously reportedmethod (Pharmacol. Biochem. Behav., 39, 541-544, 1991). STZ at a dose of200 mg/kg was intraperitoneally administered to 4-weeks-old ICR mice. Apre-drug tail pinch test was carried out in the afternoon at thefourteenth day after the STZ-administration, and animals showing areaction latency of 3 seconds or less were subjected to the next day'stest. At the fifteenth day after the STZ-administration, each drug wasadministered orally and a post-drug tail pinch test was carried out 45minutes after the drug administration.

[0075] In this connection, normal mice without STZ-treatment showed anaverage reaction latency of 6 to 7 seconds in this test. Regarding theSTZ-treated mice used in this test, animals which showed a distinctreduction of pain threshold, i.e. animals with a reaction latency of 3seconds or less were used in the evaluation of drugs.

[0076] Statistical analysis was performed with Steel test between thecontrol group and drug-administered group (* and ** in the table are asfollows; *p<0.05, **p<0.01 vs. control group).

[0077] (Results)

[0078] The compound A significantly prolonged the reaction latency at 30mg/kg po as shown in FIG. 1A and FIG. 1B, and the compound Bsignificantly prolonged the latency at 10 mg/kg po as shown in FIG. 2.

[0079] Thus, it was confirmed that compounds having mGluR1 antagonisticactivity have a therapeutic effect for neuropathic pain caused bydiabetes mellitus.

TEST EXAMPLE 3 (Spinal L5/L6 Nerve-Ligated Rats)

[0080] The test was carried out by modifying a previously reportedmethod (Pain, 50, 355-363, 1992). SD rats were used. The left sidelumbar nerves (L5 and L6) of each animal were ligated with silk threadsunder pentobarbital anesthesia. The following test was carried out 7days after the operation.

[0081] After 45 minutes of oral administration of drug, von Frey hair(VFH) test was carried out to measure a mechanical pain threshold. Themeasurement was carried out on both hindpaws.

[0082] Although there was little difference in the mechanical painthresholds between the left and right hindpaw in sham-operated rats inwhich the threshold was 17 to 20 g (log(g): 1.23-1.30), the obviousreduction of the mechanical pain threshold in the operated side hinpawwas found in spinal L5/L6 nerve-ligated rats.

[0083] Statistical analysis was performed with Dunnet test and carriedout on each side hindpaw between the control group and drug-administeredgroups (** and *** in the table are as follows; **p<0.01, ***p<0.001 vs.control group)

[0084] (Results)

[0085] Results of the VFH test are shown in FIG. 2. The compound Aimproved lowered mechanical pain thresholds in the operated side hindpawat 30 or 100 mg/kg po.

[0086] Thus, it was confirmed that a compound having mGluR1 antagonisticactivity has a therapeutic effect for neuropathic pain caused by nervecompression.

[0087] As a result of these test examples, it was confirmed thatcompounds having selective and potent antagonistic activity for mGluR1are useful as therapeutic agents for various types of neuropathic pain.

[0088] Production examples of the compounds A and B used in theinvention are shown below.

PRODUCTION EXAMPLE 16-Amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzoimidazole-2-carboxamidedihydrochloride (Compound A)

[0089] The title compound can be synthesized by the method described inProduction Method 1 of WO 99/44639.

[0090] A THF (80 ml)-methanol (30 ml) solution ofN-cyclohexyl-N,3-dimethyl-6-nitrothiazolo[3,2-a]benzoimidazole-2-carboxamide(5.35 g) was mixed at room temperature with aqueous solution (50 ml) ofsodium hydrosulfite (12.5 g) and stirred at the same temperature for 12hours. Next, this was mixed with concentrated hydrochloric acid (10 ml)and heated under reflux for 1 hour. Subsequently, THF and methanol wereevaporated under a reduced pressure, and the residue was diluted withwater and then neutralized with 28% aqueous ammonia. After extractionwith ethyl acetate, the extract was washed with water and saturatedbrine and dried with anhydrous sodium sulfate, and then the solvent wasevaporated under a reduced pressure. The residue was purified by acolumn chromatography (eluent; chloroform:methanol=20:1), converted intohydrochloride and then recrystallized from methanol-ethyl acetate,thereby obtaining the title compound (3.78 g) as light brown crystals.

[0091] NMR: (DMSO-d₆, TMS internal standard)

[0092] δ: 8.11 (d, 1H), 7.84 (d, 1H), 7.41 (dd, 1H), 4.94 (br) 3.80-4.20(br, 1H), 2.94 (s, 3H), 2.71 (s, 3H), 1.50-1.85 (m, 7H), 1.22-1.40 (m,2H), 1.02-1.18 (m, 1H).

[0093] MS (FAB): 343 (M+1).

PRODUCTION EXAMPLE 2 (Compound B)

[0094](+)-(1R,2S)-6-Amino-N-methyl-N-(2-methylcyclohexyl)thiazolo[3,2-a]benzoimidazole-2carboxamide dihydrochloride

[0095] Under ice-cooling, fuming nitric acid (0.28 ml) was added toconcentrated sulfuric acid (22 ml) solution of(+)-(1R,2S)-N-methyl-N-(2-methylcyclohexyl)thiazolo[3,2-a]benzoimidazole-2-carboxamide(2.2 g) and the mixture was stirred at the same temperature for 30minutes. The reaction solution was poured into ice water and neutralizedwith a 28% aqueous ammonia solution and then the resulting precipitatewas collected by filtration to obtain(1R,2S)-N-methyl-N-(2-methylcyclohexyl)-6-nitrothiazolo[3,2-a]benzoimidazole-2-carboxamide.This was treated in the same manner as in Production Example 1 to obtainthe title compound (242 mg).

[0096] [α]²⁵ _(D)=+17.08° (c 0.24, EtOH).

[0097] NMR: (DMSO-d₆, TMS Internal Standard)

[0098] δ: 9.16 (s, 1 H), 8.15 (s, 1 H), 7.81 (d, 1 H), 7.42 (dd, 1 H),4.85 (br), 4.25-4.38 (m, 1 H), 3.26 (s, 3 H), 1.30-2.35 (m, 9 H), 1.01(d, 3 H).

[0099] MS (FAB): 343 (M+1).

INDUSTRIAL APPLICABILITY

[0100] According to the present invention, compounds having mGluR1antagonistic activity have a therapeutic effect on the lowered painthreshold in various types of neuropathic pain with their systemicadministration, so that they are useful as pharmaceuticals forneuropathic pain, which are easy for patients to take and have efficienttherapeutic effect with less side effects.

1. A pharmaceutical composition for systemic administration for use intreating a neuropathic pain, which comprises a compound having mGluR1antagonistic activity in an amount effective for improving theneuropathic pain and a pharmaceutically acceptable carrier.
 2. Thepharmaceutical composition according to claim 1, wherein the neuropathicpain is a neuropathic pain induced by diabetes or compression of nerves.3. The pharmaceutical composition according to claim 2, wherein theneuropathic pain is a neuropathic pain induced by diabetes.
 4. Thepharmaceutical composition according to claim 1, wherein the systemicadministration method is oral administration.
 5. The pharmaceuticalcomposition according to claim 1, wherein the compound having mGluR1antagonistic activity is a compound having sufficient mGluR1 antagonismfor expressing neuropathic pain improving effect by systemicadministration.
 6. The pharmaceutical composition according to claim 1,wherein the compound having mGluR1 antagonism is a compound selectedfrom6-amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzoimidazole-2-carboxamidedihydrochloride and(+)-(1R,2S)-6-amino-N-methyl-N-(2-methylcyclohexyl)thiazolo[3,2-a]benzoimidazole-2-carboxamidedihydrochloride.